Despite the identification of insulin deficiency as the primary cause of diabetes, and the ability to readily supply this hormone to patients, diabetes continues to be a morbid and debilitating disease. There is now an increasing body of both clinical and experimental evidence that chronic, subclinical hyperglycemia, rather than the insulin deficiency itself, may be the source of the complications of this disease. It is now recognized that one of the sequelae of diabetic hyperglycemia is an increased body burden of protein carrying nonenzymatically attached glucose, and that the extent of glucosylation correlates with the severity of hyperglycemia. The trend in current thinking is that nonenzymatic glucosylation of body protein may be only the first step in a series of protein modification and crosslinking reactions which are more directly the source of the pathology of diabetes. We have developed a model system using the well characterized protein, Ribonuclease A, to study the kinetics and products of nonenzymatic glucosylation and glucose-dependent crosslinking of protein under physiological conditions in vitro. The objectives of future research with this system are: (1) to continue chemical and physical characterization of the immediate products of nonenzymatic glucosylation of protein; (2) to determine the features of protein structure and chemistry which affect the site specificity of nonenzymatic glucosylation; (3) to characterize both the mechanism of the crosslinking reaction as well as the structure of the crosslink itself; and (4) to improve or develop methods for detecting and quantitating nonenzymatic glucosylation and glucose-dependent crosslinking of protein in vitro and in vivo. We hope that a better understanding of the extent to which these glucosylation and crosslinking reactions occur in vivo under normal and pathological conditions will increase our understanding of their role in the complications of diabetes. This knowledge should provide a better basis for the design and evaluation of therapeutic approaches to the management of diabetes.